Grain-oriented electrical steel sheet and method for producing same

ABSTRACT

A grain-oriented electrical steel sheet having a composition containing, in mass %, C: 0.005% or less, Si: 2.0% to 4.5%, and Mn: 0.5% or less, and also containing Sb and P in respective ranges satisfying 0.01%≦[% Sb]≦0.20% and 0.02%≦[% P]≦2.0×[% Sb], with a balance being Fe and incidental impurities, wherein when the steel sheet is excited to 1.0 T at 50 Hz in a rolling transverse direction, a magnetizing force (TD-H 10 ) and an iron loss (TD-W 10 ) are respectively (TD-H 10 )≧200 A/m and (TD-W 10 )≧1.60 W/kg. Thus, a grain-oriented electrical steel sheet having excellent transformer core loss can be obtained industrially stably at low cost.

TECHNICAL FIELD

The disclosure relates to a grain-oriented electrical steel sheet havingexcellent iron loss property and a method for producing thegrain-oriented electrical steel sheet, and is particularly intended toobtain a grain-oriented electrical steel sheet having excellent magneticproperty at low cost.

BACKGROUND

A grain-oriented electrical steel sheet is a soft magnetic material usedas an iron core material of a transformer or generator, and has crystaltexture in which <001> orientation which is the easy magnetization axisof iron is highly accumulated into the rolling direction of the steelsheet. Such texture is formed through secondary recrystallization ofpreferentially causing the growth of giant crystal grains in (110)[001]orientation which is called Goss orientation, when secondaryrecrystallization annealing is performed in the process of producing thegrain-oriented electrical steel sheet.

A conventional process of producing such a grain-oriented electricalsteel sheet is as follows.

A slab containing about 4.5 mass % or less Si and an inhibitor componentsuch as MnS, MnSe, and AlN is heated to 1300° C. or more to dissolve theinhibitor component. The slab is then hot rolled, and optionally hotband annealed. The hot rolled sheet is then cold rolled once, or twiceor more with intermediate annealing therebetween, to final sheetthickness. The cold rolled sheet is then subjected to primaryrecrystallization annealing in a wet hydrogen atmosphere, to performprimary recrystallization and decarburization. After this, an annealingseparator having magnesia (MgO) as a main ingredient is applied to theprimary recrystallization annealed sheet, and then final annealing isperformed at 1200° C. for about 5 h to develop secondaryrecrystallization and purify the inhibitor component (for example, thespecification of U.S. Pat. No. 1,965,559 A (PTL 1), JP S40-15644 B2 (PTL2), and JP S51-13469 B2 (PTL 3)).

CITATION LIST Patent Literatures

PTL 1: U.S. Pat. No. 1,965,559 A

PTL 2: JP S40-15644 B2

PTL 3: JP S51-13469 B2

PTL 4: JP 2000-129356 A

PTL 5: JP 2004-353036 A

SUMMARY Technical Problem

As described above, the grain-oriented electrical steel sheet isconventionally manufactured by the process of containing a precipitate(inhibitor component) such as MnS, MnSe, and AlN in the slab stage,heating the slab at a high temperature exceeding 1300° C. to dissolvethe inhibitor component, and causing fine precipitation in thesubsequent step to develop secondary recrystallization. Thus,high-temperature slab heating exceeding 1300° C. is necessary in theconventional grain-oriented electrical steel sheet manufacturingprocess, which requires very high manufacturing cost. The conventionalprocess therefore has a problem of being unable to meet the recentdemands to reduce manufacturing costs.

To solve this problem, a technique (inhibitorless method) for enablingsecondary recrystallization without containing any inhibitor componentwas developed in JP 2000-129356 A (PTL 4). This method involves acompletely different technical idea from that of the conventionalgrain-oriented electrical steel sheet producing method.

In detail, while the conventional grain-oriented electrical steel sheetproducing method develops secondary recrystallization using aprecipitate (inhibitor) such as MnS, AlN, and MnSe, the inhibitorlessmethod develops secondary recrystallization by texture (texture control)through high purification without using such an inhibitor. Theinhibitorless method does not require high-temperature slab heating andhigh-temperature and long-duration secondary recrystallizationannealing, and so enables the manufacture of the grain-orientedelectrical steel sheet at low cost.

However, the inhibitorless method cannot necessarily achieve sufficientmagnetic property and stability, although it is advantageous in that thegrain-oriented electrical steel sheet can be manufactured at low cost.

To solve this problem, we further studied the technique of developingsecondary recrystallization without containing any inhibitor componentin the slab. As a result, we developed and proposed a technique(sulfurization method) that can stably develop secondaryrecrystallization by increasing the amount of S in the steel substrateafter the primary recrystallization annealing and before the completionof secondary recrystallization, even in the case where no inhibitorcomponent is contained in the slab (JP 2004-353036 A (PTL 5)).

By increasing the amount of S in the steel substrate using thesulfurization method, the amount of S segregating to grain boundariesincreases, as a result of which the movement of grain boundariessurrounding orientations other than the Goss orientation is furthersuppressed. This stabilizes secondary recrystallization, and enhancesthe sharpness of secondary grains to the Goss orientation, with it beingpossible to improve magnetic property.

However, there is a problem in that the addition of a large amount ofsulfurization agent causes excessive oxidation reaction during thesecondary recrystallization annealing, and leads to the formation of abase film defective part called sparkle or frost.

As is well known, grain-oriented electrical steel sheets are mainly usedas transformer iron cores. Transformers are broadly classified intostacked iron core transformers and wound iron core transformers,depending on their iron core structures.

A stacked iron core transformer has an iron core formed by stackingsteel sheets cut to a desired shape. A wound iron core transformer hasan iron core formed by winding a steel strip slit to a desired width. Aslarge-capacity transformers, stacked iron core transformers are usedexclusively.

An important property required of transformers is transformer core loss.Transformer core loss is energy loss when a transformer iron core isexcited. Higher transformer core loss leads to higher power loss, and sotransformer core loss needs to be as low as possible.

However, iron loss can degrade as a result of processing a steel sheetinto a transformer.

For example, there are instances where low iron loss cannot bemaintained when pinch rolls for conveying the steel sheet or measuringrolls for measuring the length of the steel sheet are pressed againstthe steel sheet.

In such instances, even when a transformer is formed using a steel sheetwhose iron loss is lowered by a magnetic domain refining effect such asa linear flaw, its iron loss value may not be as low as expected.Particularly in the case of using the steel sheet for a stacked ironcore transformer, stress relief annealing is not performed afterprocessing the steel sheet into the iron core, and so problems such asdegraded iron loss and increased noise can arise.

The conventional technique of adding a sulfurization agent hasdifficulty in forming the base film, and therefore has a problem in thatthe influence of strain associated with processing into a transformer issignificant and transformer core loss degrades.

It could be helpful to propose a grain-oriented electrical steel sheetthat has excellent magnetic property and can be produced at low costwith no need for high-temperature slab heating and whose transformercore loss is effectively improved by reducing the influence of strainassociated with processing into a transformer, and an advantageousmethod for producing the grain-oriented electrical steel sheet.

Solution to Problem

We closely examined the technique of developing secondaryrecrystallization without containing any inhibitor component in a slaband improving magnetic property by sulfurization treatment.

As a result, we developed a technique that can stably realize favorablebase film formation by optimizing material components even in the casewhere sulfurization treatment is performed.

The following describes the experimental results that led to thedisclosure. In the following description, “%” with regard to componentsdenotes mass % unless otherwise stated.

Experiment 1

A silicon steel slab containing, in mass %, Si: 3.3%, C: 0.03%, Mn:0.07%, S: 0.002%, Al: 0.006%, and N: 0.003% and further containing P andSb in the ranges of P: 0% to 0.2% and Sb: 0% to 0.2% was heated at 1220°C. for 30 minutes, and then hot rolled to obtain a hot rolled sheet of2.5 mm in thickness. The hot rolled sheet was hot band annealed at 1025°C. for 1 minute, and then cold rolled to final sheet thickness.

Following this, the cold rolled sheet was primary recrystallizationannealed, and then an annealing separator having MgO as a mainingredient and containing 10% magnesium sulfate was applied at 12.5 g/m²to the primary recrystallized sheet and dried. The primaryrecrystallized sheet was then secondary recrystallization annealed underthe following condition: heating rate: 15° C./h, atmosphere gas: N₂ gasup to 900° C., and H₂ gas at 900° C. or more, and soaking treatment:1160° C. for 5 h.

FIG. 1 illustrates the result of studying the relationship between theadditive amount of P, the additive amount of Sb, and the magnetic fluxdensity.

As illustrated in FIG. 1, in the case of adding P singly, the magneticflux density improving effect was poor, and rather the magnetic fluxdensity tended to degrade by the addition of P. In the case of addingSb, the magnetic flux density increased with the addition of P until theadditive amount of P reached the additive amount of Sb, and graduallydecreased once the additive amount of P exceeded the additive amount ofSb. This demonstrates that the magnetic flux density improving effect bythe addition of P is achieved by adding Sb up to about the same amountas P.

Although the effect of adding P and Sb in combination is not clear, weassume the following:

P is a grain boundary segregation element, and has a function ofsuppressing recrystallization nucleation from the grain boundaries andfacilitating recrystallization nucleation from inside the grains toincrease the Goss orientation in the primary recrystallized texture. Pthus has an effect of stabilizing secondary recrystallization nucleationand improving magnetic property. However, the addition of P has anadverse effect of facilitating surface oxidation during the secondaryrecrystallization annealing and hindering the sulfurization effect andalso hindering normal base film formation.

Sb is a surface segregation element, and has a function of suppressingoxidation during the secondary recrystallization annealing to optimizethe oxidation quantity and stabilize secondary recrystallization andbase film formation. Sb thus has an effect of lessening the adverseeffect of the addition of P.

Accordingly, adding Sb and P in combination is very effective inachieving the aforementioned texture improving effect by the addition ofP.

The phenomenon of improving magnetic property by sulfurization isspecific to the case where the steel slab contains no inhibitorcomponent. In the case where there is no inhibitor (precipitate) such asAlN and MnS in the steel, the grain boundaries surrounding theGoss-oriented grains in the primary recrystallized texture have highermobility than the grain boundaries surrounding the grains in the otherorientations, as a result of which the Goss orientation undergoespreferential growth (secondary recrystallization).

Although the reason why magnetic property is improved by increasing theamount of S in the steel substrate after primary recrystallization isnot clear, we assume the following:

When the amount of S in the steel substrate is increased, the amount ofS segregating to grain boundaries increases, as a result of which themovement of the grain boundaries surrounding the orientations other thanthe Goss orientation is further suppressed. This stabilizes secondaryrecrystallization, and enhances the sharpness of secondary grains to theGoss orientation. The coexistence of P and S which are elements having astrong tendency to segregate to grain boundaries further enhances themagnetic property improving effect.

Moreover, regarding the method of reducing the influence of strainassociated with processing into a transformer, we studied the conditionof the flattening annealing atmosphere, and discovered that the magneticproperty in the rolling transverse direction (the direction orthogonalto the rolling direction) changes, and there is a very high correlationbetween the magnetic property in the transverse direction and theinfluence of strain associated with processing into a transformer.

We then discovered a preferable range of the magnetic property in thetransverse direction effective in reducing the influence of strain, asdescribed below.

Experiment 2

A silicon steel slab containing, in mass %, Si: 3.3%, C: 0.03%, Mn:0.07%, S: 0.002%, Al: 0.006%, N: 0.003%, P: 0.05%, and Sb: 0.05% washeated at 1220° C. for 30 minutes, and then hot rolled to obtain a hotrolled sheet of 2.5 mm in thickness. The hot rolled sheet was hot bandannealed at 1025° C. for 1 minute, and then cold rolled to final sheetthickness.

Following this, the cold rolled sheet was primary recrystallizationannealed, and then an annealing separator having MgO as a mainingredient and containing 10% magnesium sulfate was applied at 12.5 g/m²to the primary recrystallized sheet and dried. The primaryrecrystallized sheet was then secondary recrystallization annealed underthe following condition: heating rate: 15° C./h, atmosphere gas: N₂ gasup to 900° C., and H₂ gas at 900° C. or more, and soaking treatment:1160° C. for 5 h.

Further, an insulating coating mainly composed of colloidal silica andmagnesium phosphate was applied. An experiment of changing the soakingtemperature (soaking time of 10 s) and the H₂ partial pressure (the restbeing N₂ atmosphere) in the annealing atmosphere in flattening annealingwas then conducted under the conditions shown in Table 1.

The magnetic property of the obtained product in each of the rollingdirection and the transverse direction was measured. In the rollingdirection, the iron loss (W_(17/50)) when exciting the product to 1.7 Tat 50 Hz was measured. In the transverse direction, the magnetizingforce (TD-H₁₀) and iron loss (TD-W₁₀) when exciting the product to 1.0 Tat 50 Hz were measured. Strain sensitivity was evaluated based on thechange (ΔW) in iron loss W_(17/50) value when passing the sheet whilepressing it by measuring rolls, which were made up of steel rolls of 100mm in diameter and 50 mm in width, with a rolling reduction force of 1.5MPa (15 kgf/cm).

Table 1 shows the obtained results.

TABLE 1 Annealing H₂ partial temperature pressure TD-H₁₀ TD-W₁₀W_(17/50) ΔW No. (° C.) (%) (A/m) (W/kg) (W/kg) (W/kg) 1 780 0.5 1721.01 0.945 0.055 2 790 0.5 180 1.05 0.935 0.035 3 800 0.5 192 1.11 0.9120.018 4 810 0.5 202 1.17 0.900 0.006 5 820 0.5 240 1.33 0.892 0.003 6830 0.5 253 1.63 0.882 0.003 7 840 0.5 255 1.72 0.874 0.004 8 850 0.5260 1.95 0.871 0.003 9 860 0.5 258 1.98 0.875 0.002 10 850 0 156 1.940.883 0.102 11 850 0.1 191 1.93 0.880 0.020 12 850 0.2 222 1.96 0.8730.005 13 850 0.5 260 1.95 0.871 0.001 14 850 1 258 1.93 0.874 0.003 15850 3 256 1.95 0.871 0.002 16 850 5 265 1.94 0.868 0.001 17 850 10 2601.94 0.873 0.003

FIGS. 2A and 2B summarize the influence of the flattening annealingtemperature on the iron loss (W_(17/50)) in the rolling direction andthe influence of the H₂ partial pressure in the annealing atmosphere onthe iron loss (W_(17/50)) in the rolling direction.

As illustrated in FIGS. 2A and 2B, the flattening annealing temperaturesignificantly influences the iron loss (W_(17/50)) in the rollingdirection, and needs to be 830° C. or more to improve the iron loss.Meanwhile, the H₂ partial pressure in the flattening annealingatmosphere hardly influences the iron loss (W_(17/50)).

FIGS. 3A and 3B summarize the influence of the flattening annealingtemperature on the iron loss degradation (ΔW) upon measuring rollrolling reduction and the influence of the H₂ partial pressure in theannealing atmosphere on the iron loss degradation (ΔW) upon measuringroll rolling reduction.

As illustrated in FIGS. 3A and 3B, the flattening annealing temperaturesignificantly influences the iron loss degradation (ΔW) upon measuringroll rolling reduction, and needs to be 820° C. or more to reduce theiron loss degradation. The H₂ partial pressure in the flatteningannealing atmosphere also significantly influences the iron lossdegradation (ΔW), and the iron loss degradation (ΔW) upon measuring rollrolling reduction is very high in the case where the hydrogen atmosphereis not introduced.

FIGS. 4A and 4B summarize the influence of the flattening annealingtemperature on the iron loss (TD-W₁₀) in the transverse direction andthe influence of the H₂ partial pressure in the annealing atmosphere onthe iron loss (TD-W₁₀) in the transverse direction.

As illustrated in FIGS. 4A and 4B, the flattening annealing temperaturesignificantly influences the iron loss (TD-W₁₀) in the transversedirection, and a higher flattening annealing temperature increases theiron loss (TD-W₁₀) in the transverse direction. Meanwhile, the H₂partial pressure in the flattening annealing atmosphere hardlyinfluences the iron loss (TD-W₁₀) in the transverse direction.

FIGS. 5A and 5B summarize the influence of the flattening annealingtemperature on the magnetizing force (TD-H₁₀) in the transversedirection and the influence of the H₂ partial pressure in the annealingatmosphere on the magnetizing force (TD-H₁₀) in the transversedirection.

As illustrated in FIGS. 5A and 5B, the flattening annealing temperaturesignificantly influences the magnetizing force (TD-H₁₀) in thetransverse direction, and a higher flattening annealing temperatureincreases the magnetizing force (TD-H₁₀) in the transverse direction.The hydrogen atmosphere also significantly influences the magnetizingforce (TD-H₁₀) in the transverse direction, and the magnetizing force(TD-H₁₀) in the transverse direction decreases in the case where thehydrogen atmosphere is not introduced.

The aforementioned experiment revealed that the flattening annealingtemperature and the hydrogen partial pressure in the flatteningannealing atmosphere influenced the iron loss (W_(17/50)) in the rollingdirection, the iron loss degradation (ΔW) upon measuring roll rollingreduction, the iron loss (TD-W₁₀) in the transverse direction, and themagnetizing force (TD-H₁₀) in the transverse direction. We then studiedtheir correlations.

FIG. 6 illustrates the result of studying the relationship between theiron loss (TD-W₁₀) in the transverse direction and the iron loss(W_(17/50)) in the rolling direction.

As illustrated in FIG. 6, when the iron loss in the transverse directionincreases, the iron loss in the rolling direction decreases. Setting theiron loss (TD-W₁₀) in the transverse direction to 1.6 W/kg or more iseffective in improving the iron loss (W_(17/50)) in the rollingdirection. As illustrated in FIG. 4A, the iron loss in the transversedirection increases with an increase in flattening annealingtemperature. In the case where there is residual strain by shapeadjustment, the stability of the 180° magnetic domain structuredecreases, which is likely to cause an increase in iron loss in thetransverse direction.

In other words, the iron loss in the transverse direction serves as anindex of residual strain.

The results in FIGS. 6 and 4A indicate that, to improve the iron loss inthe rolling direction, the flattening annealing temperature needs to be830° C. or more so that the iron loss in the transverse direction is 1.6W/kg or more.

FIG. 7 illustrates the relationship between the magnetizing force(TD-H₁₀) in the transverse direction and the iron loss degradation (ΔW)in the rolling direction.

As illustrated in FIG. 7, when the magnetizing force in the transversedirection increases, the iron loss degradation (ΔW) upon measuring rollrolling reduction decreases. The magnetizing force in the transversedirection increases by increasing the flattening annealing temperatureand introducing the hydrogen atmosphere, as illustrated in FIGS. 5A and5B.

In other words, the magnetizing force in the transverse direction servesas an index of film tension.

The results in FIGS. 7 and 5A and 5B indicate that, to limit the ironloss degradation (ΔW) upon measuring roll rolling reduction to a lowlevel of 0.01 W/kg or less, the flattening annealing temperature needsto be 810° C. or more and preferably 830° C. or more and 0.30% or morehydrogen needs to be introduced into the flattening annealing atmosphereso that the magnetizing force (TD-H₁₀) in the transverse direction is200 A/m or more.

It is assumed that, by increasing the flattening annealing temperatureand introducing the hydrogen atmosphere, the water content in thecoating mainly composed of phosphate is reduced to strengthen thecoating film tension.

The disclosure is based on the results of the two experiments describedabove and further studies.

In detail, we provide the following:

1. A grain-oriented electrical steel sheet having a compositioncontaining (consisting of), in mass %, C: 0.005% or less, Si: 2.0% to4.5%, and Mn: 0.5% or less, and also containing Sb and P in respectiveranges satisfying 0.01%≦[% Sb]≦0.20% and 0.02%≦[% P]≦2.0×[% Sb], with abalance being Fe and incidental impurities,

wherein when the steel sheet is excited to 1.0 T at 50 Hz in a rollingtransverse direction, a magnetizing force (TD-H₁₀) and an iron loss(TD-W₁₀) are respectively (TD-H₁₀)≧200 A/m and (TD-W₁₀)≧1.60 W/kg.

2. The grain-oriented electrical steel sheet according to 1, wherein thecomposition further contains, in mass %, one or more selected from Ni:0.005% to 1.50%, Sn: 0.03% to 0.20%, Cu: 0.02% to 0.50%, Cr: 0.02% to0.50%, Mo: 0.01% to 0.50%, and Nb: 0.002% to 0.01%.

3. A method for producing a grain-oriented electrical steel sheet,comprising:

providing a steel slab having a composition containing, in mass %, C:0.08% or less, Si: 2.0% to 4.5%, and Mn: 0.5% or less, containing eachof S, Se, and O: less than 50 ppm, N: less than 60 ppm, and sol.Al: lessthan 100 ppm, and also containing Sb and P in respective rangessatisfying 0.01%≦[% Sb]≦0.20% and 0.02%≦[% P]≦2.0×[% Sb], with a balancebeing Fe and incidental impurities;

optionally reheating the steel slab;

thereafter hot rolling the steel slab to obtain a hot rolled sheet;

optionally hot band annealing the hot rolled sheet;

thereafter cold rolling the hot rolled sheet either once, or twice ormore with intermediate annealing performed therebetween, to obtain acold rolled sheet having a final sheet thickness;

thereafter performing decarburization and primary recrystallizationannealing on the cold rolled sheet, to obtain a decarburization andprimary recrystallization annealed sheet;

thereafter applying an annealing separator mainly composed of MgO to thedecarburization and primary recrystallization annealed sheet;

thereafter performing secondary recrystallization annealing on thedecarburization and primary recrystallization annealed sheet, to obtaina secondary recrystallization annealed sheet; and

further performing flattening annealing on the secondaryrecrystallization annealed sheet,

wherein 2.0 mass % to 15.0 mass % magnesium sulfate is contained in theannealing separator,

the flattening annealing is performed at a temperature of 830° C. ormore in an atmosphere having a H₂ partial pressure of 0.3% or more, and

when the steel sheet is excited to 1.0 T at 50 Hz in a rollingtransverse direction, a magnetizing force (TD-H₁₀) and an iron loss(TD-W₁₀) are respectively (TD-H₁₀)≧200 A/m and (TD-W₁₀)≧1.60 W/kg.

Advantageous Effect

It is thus possible to produce a grain-oriented electrical steel sheethaving excellent transformer core loss industrially stably at low cost,which is of great industrial value.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a graph illustrating the relationship between the additiveamount of P, the additive amount of Sb, and the magnetic flux density;

FIG. 2A is a graph illustrating the influence of the flatteningannealing temperature on the iron loss (W_(17/50)) in the rollingdirection;

FIG. 2B is a graph illustrating the influence of the H₂ partial pressurein the annealing atmosphere on the iron loss (W_(17/50)) in the rollingdirection;

FIG. 3A is a graph illustrating the influence of the flatteningannealing temperature on the iron loss degradation (ΔW) upon measuringroll rolling reduction;

FIG. 3B is a graph illustrating the influence of the H₂ partial pressurein the annealing atmosphere on the iron loss degradation (ΔW) uponmeasuring roll rolling reduction;

FIG. 4A is a graph illustrating the influence of the flatteningannealing temperature on the iron loss (TD-W₁₀) in the transversedirection;

FIG. 4B is a graph illustrating the influence of the H₂ partial pressurein the annealing atmosphere on the iron loss (TD-W₁₀) in the transversedirection;

FIG. 5A is a graph illustrating the influence of the flatteningannealing temperature on the magnetizing force (TD-H₁₀) in thetransverse direction;

FIG. 5B is a graph illustrating the influence of the H₂ partial pressurein the annealing atmosphere on the magnetizing force (TD-H₁₀) in thetransverse direction;

FIG. 6 is a graph illustrating the relationship between the iron loss(TD-W₁₀) in the transverse direction and the iron loss (W_(17/50)) inthe rolling direction; and

FIG. 7 is a graph illustrating the relationship between the magnetizingforce (TD-H₁₀) in the transverse direction and the iron loss degradation(ΔW) in the rolling direction.

DETAILED DESCRIPTION

One of the disclosed embodiments is described in detail below.

The reasons for limiting the chemical composition of a steel slab to theaforementioned range in this embodiment are described first.

C: 0.08% or less

C is an element useful in improving primary recrystallized texture. Ifthe C content is more than 0.08%, however, the primary recrystallizedtexture degrades. The C content is therefore limited to 0.08% or less.The C content is desirably in the range of 0.01% to 0.06%, in terms ofmagnetic property. In the case where the required level of magneticproperty is not so high, the C content may be 0.01% or less in order toomit or simplify decarburization in primary recrystallization annealing.

Moreover, it is essential to reduce the C content to 0.005% or less inthe steel sheet after final annealing, in order to prevent magneticaging.

Si: 2.0% to 4.5%

Si is an element useful in improving iron loss by increasing electricalresistance, and so the Si content is 2.0% or more. If the Si content ismore than 4.5%, however, cold rolling manufacturability decreasessignificantly. The upper limit of the Si content is therefore 4.5%. Theaddition of Si may be omitted depending on the required iron loss level.

Mn: 0.5% or less

Mn has an effect of improving hot workability during manufacture.

If the Mn content is more than 0.5%, however, the primary recrystallizedtexture deteriorates and leads to lower magnetic property. The Mncontent is therefore limited to 0.5% or less. The lower limit of the Mncontent is preferably 0.05%.

S, Se, and O: less than 50 ppm each

If the content of each of S, Se, and O is 50 ppm or more, secondaryrecrystallization is difficult. This is because a coarse oxide or MnS orMnSe coarsened due to slab heating makes the primary recrystallizedtexture non-uniform. The content of each of S, Se, and O is thereforelimited to less than 50 ppm.

N: less than 60 ppm

Excessive N also makes secondary recrystallization difficult, as with S,Se, and O. Particularly if the N content is 60 ppm or more, secondaryrecrystallization is unlikely to occur, and the magnetic propertydegrades. The N content is therefore limited to less than 60 ppm.

sol.Al: less than 100 ppm

Excessive Al also makes secondary recrystallization difficult.Particularly if the sol.Al content is 100 ppm or more, secondaryrecrystallization is unlikely to occur under the low-temperature slabheating condition, and the magnetic property degrades. Al is thereforelimited to less than 100 ppm in sol.Al content.

Sb and P: 0.01%≦[% Sb]≦0.20% and 0.02%≦[% P]≦2.0×[% Sb] respectively

In this embodiment, it is important to contain Sb and P in combinationin these respective ranges. By adding Sb and P in combination in theseranges, the desired sulfurization effect in this embodiment iseffectively achieved, and magnetic property degradation due to surfaceoxidation is suppressed. As a result, favorable magnetic property andbase film property can be obtained throughout the coil length. If the Sbcontent or the P content is less than the aforementioned range, theeffect cannot be achieved. If the Sb content or the P content is morethan the aforementioned range, not only the magnetic property degrades,but also the formation of the base film is difficult.

While the essential components have been described above, the followingelements may be contained as appropriate as components for improving themagnetic property industrially more stably in this embodiment.

Ni: 0.005% to 1.50%

Ni has a function of improving the magnetic property by enhancing theuniformity of the hot rolled sheet texture. To do so, the Ni content ispreferably 0.005% or more. If the Ni content is more than 1.50%,secondary recrystallization is difficult, and the magnetic propertydegrades. Accordingly, the Ni content is desirably in the range of0.005% to 1.50%.

Sn: 0.03% to 0.20%

Sn has a function of suppressing the nitriding or oxidation of the steelsheet during secondary recrystallization annealing and promoting thesecondary recrystallization of crystal grains having favorable crystalorientation to effectively improve the magnetic property, in particularthe iron loss property. To do so, the Sn content is preferably 0.03% ormore. If the Sn content is more than 0.20%, cold rollingmanufacturability decreases. Accordingly, the Sn content is desirably inthe range of 0.03% to 0.20%.

Cu: 0.02% to 0.50%

Cu is a useful element that suppresses the nitriding or oxidation of thesteel sheet during secondary recrystallization annealing and promotesthe secondary recrystallization of crystal grains having favorablecrystal orientation to effectively improve the magnetic property. To doso, the Cu content is preferably 0.02% or more. If the Cu content ismore than 0.50%, cold rolling manufacturability decreases. Accordingly,the Cu content is desirably in the range of 0.02% to 0.50%.

Cr: 0.02% to 0.50%

Cr has a function of stabilizing the formation of the forsterite basefilm. To do so, the Cr content is preferably 0.02% or more. If the Crcontent is more than 0.50%, secondary recrystallization is difficult,and the magnetic property degrades. Accordingly, the Cr content isdesirably in the range of 0.02% to 0.50%.

Mo: 0.01% to 0.50%

Mo has a function of suppressing high-temperature oxidation and reducingsurface defects called scab. To do so, the Mo content is preferably0.01% or more. If the Mo content is more than 0.50%, cold rollingmanufacturability decreases. Accordingly, the Mo content is desirably inthe range of 0.01% to 0.50%.

Nb: 0.002% to 0.01%

Nb is a useful element that inhibits the growth of primaryrecrystallized grains and promotes the secondary recrystallization ofcrystal grains having favorable crystal orientation to improve themagnetic property. To do so, the Nb content is desirably 0.002% or more.If the Nb content is more than 0.01%, Nb remains in the steel substrateand degrades the iron loss. Accordingly, the Nb content is desirably inthe range of 0.002% to 0.01%.

The following describes a production method in this embodiment.

A steel slab adjusted to the aforementioned chemical composition rangeis, after or without being reheated, hot rolled. In the case ofreheating the slab, the reheating temperature is desirably about 1000°C. or more and 1300° C. or less. Slab heating exceeding 1300° C. ismeaningless in this embodiment in which the slab contains no inhibitor,and not only causes an increase in cost but also significantly degradesthe magnetic property due to the growth of giant crystal grains. If thereheating temperature is less than 1000° C., the rolling load increases,making the rolling difficult.

Following this, the hot rolled sheet is optionally hot band annealed.The hot rolled sheet is then cold rolled once, or twice or more withintermediate annealing therebetween, to obtain a final cold rolledsheet. The cold rolling may be performed at normal temperature.Alternatively, the cold rolling may be warm rolling with the steel sheettemperature being higher than normal temperature, e.g. about 250° C.

The final cold rolled sheet is then subjected to decarburization/primaryrecrystallization annealing. A first objective of thedecarburization/primary recrystallization annealing is to cause theprimary recrystallization of the cold rolled sheet having rolledmicrostructure to adjust it to an optimal primary recrystallized texturefor secondary recrystallization. For this objective, the annealingtemperature in the primary recrystallization annealing is desirablyabout 800° C. or more and less than about 950° C. The annealingatmosphere is desirably a wet hydrogen nitrogen atmosphere or a wethydrogen argon atmosphere.

A second objective is decarburization. If more than 0.005% carbon iscontained in the product sheet, the iron loss degrades. The carboncontent is therefore desirably reduced to 0.005% or less.

A third objective is to form a subscale made up of an internal oxidationlayer of SiO₂ which is the raw material of the base film mainly composedof forsterite. If the upstream-stage temperature of decarburizationannealing is less than 800° C., oxidation reaction and decarburizationreaction do not progress sufficiently, and necessary oxidation anddecarburization cannot be completed.

After the decarburization/primary recrystallization annealing, anannealing separator mainly composed of magnesia (MgO) is applied to thesurface of the steel sheet. Here, magnesium sulfate is added to theannealing separator mainly composed of MgO, in order to improve themagnetic property by the sulfurization treatment of increasing theamount of S in the steel substrate after the primary recrystallizationannealing and before the completion of secondary recrystallization.

If the additive amount of magnesium sulfate is less than 2.0%, themagnetic property improving effect is insufficient. If the additiveamount of magnesium sulfate is more than 15.0%, the grain growth issuppressed excessively, and the magnetic property improving effect isinsufficient and also the formation of the base film is adverselyaffected.

The expression “mainly composed of magnesia” in this embodiment meansthat 50% or more magnesia is contained in the annealing separator.Sub-components such as Na₂S₂O₃ and TiO₂ may be added to the annealingseparator in small amounts, according to conventional methods.

After this, secondary recrystallization annealing is performed. Duringthe secondary recrystallization annealing, magnesium sulfate decomposesand exerts the sulfurization effect, thus realizing crystal texturehighly aligned with the Goss orientation. Favorable magnetic propertycan be obtained in this way.

The secondary recrystallization annealing is effectively performed bydiffusing S into the steel substrate with a heating rate of 30° C./H orless, as disclosed in JP 4321120 B. The annealing atmosphere may be anyof N₂, Ar, and mixed gas thereof. Here, H₂ is not used as atmosphere gasuntil the completion of secondary recrystallization. This is because Sin the annealing separator goes out of the system as H₂S (gas), causinglower sulfurization effect especially in the coil edges.

After the secondary recrystallization annealing, an insulating coatingis further applied to the surface of the steel sheet and baked. The typeof the insulating coating is not particularly limited, and may be anyconventionally well-known insulating coating. For example, a method ofapplying an application liquid containing phosphate-chromate-colloidalsilica described in JP S50-79442 A and JP S48-39338 A to the steel sheetand baking it to also perform flattening annealing is preferable.

Flattening annealing is then performed. This flattening annealing isimportant in this embodiment.

The flattening annealing temperature needs to be 830° C. or more. If theflattening annealing temperature is less than 830° C., strain for shapeadjustment remains, which decreases the iron loss in the TD directionand simultaneously degrades the iron loss in the RD direction. The ironloss in the TD direction for preventing degradation in the iron loss inthe RD direction in the product sheet is 1.60 W/kg or more.

Moreover, 0.30% or more hydrogen needs to be introduced into theflattening annealing atmosphere. If the hydrogen partial pressure in theatmosphere is less than 0.30%, the coating film tension decreases, andthe magnetizing force in the TD direction decreases. This results inhigher degradation of transformer core loss due to the application ofstrain associated with processing into a transformer. To reduce the ironloss degradation caused by the application of strain associated withprocessing into a transformer and improve the transformer core loss, themagnetizing force when exciting the product sheet to 1.0 T in the TDdirection needs to be 200 A/m or more.

EXAMPLES Example 1

A continuously cast slab having a composition containing C: 0.03%, Si:3.5%, Mn: 0.08%, sol.Al: 75 ppm, N: 45 ppm, S: 30 ppm, Se: 1 ppm, O: 9ppm, P: 0.06%, and Sb: 0.10% with the balance being Fe and incidentalimpurities was reheated to 1230° C., and then hot rolled to obtain a hotrolled sheet of 2.5 mm in sheet thickness. The hot rolled sheet was thenhot band annealed at 1050° C. for 10 s, and subsequently cold rolled at200° C. to a sheet thickness of 0.27 mm. The cold rolled sheet was thensubjected to primary recrystallization annealing also serving asdecarburization at 850° C. for 120 s in an atmosphere of H₂: 55%, N₂:45%, and dew point: 55° C., with the heating rate from 500° C. to 700°C. being 20° C./s. The C content after this annealing was 30 ppm.

A sample was collected from the obtained primary recrystallizationannealed sheet, and an annealing separator having MgO as a mainingredient and containing magnesium sulfate in the proportion shown inTable 2 was applied at 12.5 g/m² to the sheet surface and dried. Thesample was then subjected to secondary recrystallization annealing underthe condition of heating to 800° C. at a heating rate of 15° C./h,heating from 800° C. to 850° C. at a heating rate of 2.0° C./h,retaining at 850° C. for 50 h, and then heating to 1160° C. at a heatingrate of 5.0° C./h and soaking for 5 h. As the atmosphere gas, N₂ gas wasused up to 850° C., and H₂ gas was used at 850° C. or more.

A treatment liquid containing phosphate-chromate-colloidal silica at amass ratio of 3:1:3 was applied to the surface of the secondaryrecrystallization annealed sheet obtained under the aforementionedcondition, and subsequently flattening annealing was performed under thecondition shown in Table 2.

The magnetic property of the obtained product sheet was then examined.The magnetic property was evaluated based on the magnetic flux densityB₈ when exciting the sheet at 800 A/m in the rolling direction and theiron loss W_(17/50) when exciting the sheet to 1.7 T at 50 Hz in analternating magnetic field in the rolling direction, the magnetizingforce (TD-H₁₀) and iron loss (TD-W₁₀) when exciting the sheet to 1.0 Tat 50 Hz in the transverse direction, and the strain sensitivity.

The strain sensitivity was evaluated based on the change (ΔW) in ironloss W_(17/50) value when passing the sheet while pressing it bymeasuring rolls, which were made up of steel rolls of 100 mm in diameterand 50 mm in width, with a rolling reduction force of 1.5 MPa (15kgf/cm).

Table 2 shows the obtained results. Magnetic flux density B₈ of 1.94 Tor more, iron loss W_(17/50) of 0.82 W/kg or less, and ΔW of 0.005 W/kgor less are regarded as excellent properties.

TABLE 2 Flattening annealing condition Additive amount of Soaking H₂partial pressure in Magnetic property of product sheet magnesium sulfatetemperature atmosphere B₈ W_(17/50) TD-H₁₀ TD-W₁₀ ΔW No. (%) (° C.) (%)(T) (W/kg) (A/m) (W/kg) (W/kg) Remarks 1 2.5 850 3.0 1.943 0.812 2711.903 0.005 Example 2 5.0 850 3.0 1.955 0.784 280 1.993 0.004 Example 310.0  850 3.0 1.960 0.775 290 2.011 0.003 Example 4 10.0  880 3.0 1.9570.763 272 2.028 0.004 Example 5 0   850 3.0 1.911 0.880 188 1.774 0.023Comparative Example 6 20.0  850 3.0 1.868 1.050 292 2.044 0.003Comparative Example 7 5.0 800 3.0 1.953 0.883 280 1.503 0.004Comparative Example 8 5.0 850 0   1.956 0.788 180 1.983 0.028Comparative Example

As is clear from Table 2, by using the material containing P and Sb incombination, applying the annealing separator mainly composed of MgO andcontaining 2.0% or more magnesium sulfate, and performing secondaryrecrystallization annealing according to the disclosure, favorablemagnetic flux density was obtained. Moreover, by setting the flatteningannealing temperature to 830° C. or more, the iron loss in the TDdirection was 1.60 W/kg or more, resulting in favorable iron loss in therolling direction. Further, by introducing 0.30% or more a hydrogenatmosphere into the flattening annealing atmosphere, the magnetizingforce when exciting the sheet to 1.0 T in the transverse direction wasensured to be 200 A/m or more, as a result of which the iron lossdegradation caused by the application of strain associated withprocessing into a transformer was reduced.

Example 2

A continuously cast slab composed of various components shown in Table 3was reheated to 1230° C., and then hot rolled to obtain a hot rolledsheet of 2.2 mm in sheet thickness. The hot rolled sheet was then hotband annealed at 1050° C. for 10 s, and subsequently cold rolled at 200°C. to a sheet thickness of 0.23 mm. The cold rolled sheet was thensubjected to decarburization annealing at 850° C. for 120 s in anatmosphere of H₂: 55%, N₂: 45%, and dew point: 55° C., with the heatingrate from 500° C. to 700° C. being 20° C./s. The C content after thedecarburization annealing was 30 ppm.

A sample was collected from the decarburization annealed sheet, and anannealing separator having MgO as a main ingredient and containingmagnesium sulfate in the proportion shown in Table 4 was applied at 12.5g/m² to the sheet surface and dried. The sample was then subjected tosecondary recrystallization annealing under the condition of heating to800° C. at a heating rate of 15° C./h, heating from 800° C. to 850° C.at a heating rate of 2.0° C./h, retaining at 850° C. for 50 h, and thenheating to 1160° C. at a heating rate of 5.0° C./h and soaking for 5 h.As the atmosphere gas, N₂ gas was used up to 850° C., and H₂ gas wasused at 850° C. or more.

A treatment liquid containing phosphate-chromate-colloidal silica at amass ratio of 3:1:3 was applied to the surface of the secondaryrecrystallization annealed sheet obtained under the aforementionedcondition, and subsequently flattening annealing was performed under thecondition shown in Table 4.

The magnetic property of the obtained product sheet was then examined.The method of evaluating the magnetic property is the same as that inExample 1.

Table 4 shows the obtained results.

TABLE 3 Chemical composition (mass %) No. C Si Mn Sb P S Se O Al NOthers Remarks 1 0.03 3.3 0.07 0.052 0.055 0.001 0.001 0.001 0.003 0.003— Conforming steel 2 0.04 3.2 0.08 0.066 0.075 0.002 0.001 0.001 0.0040.002 Ni: 0.30 Conforming steel 3 0.02 3.2 0.08 0.036 0.055 0.002 0.0010.001 0.004 0.002 Sn: 0.10 Conforming steel 4 0.03 3.4 0.11 0.044 0.0450.001 0.001 0.001 0.005 0.003 Cu: 0.10 Conforming steel 5 0.04 3.3 0.060.078 0.077 0.002 0.001 0.001 0.006 0.001 Cr: 0.08 Conforming steel 60.03 3.1 0.07 0.055 0.058 0.001 0.001 0.001 0.004 0.003 Mo: 0.1Conforming steel 7 0.02 3.2 0.08 0.060 0.050 0.002 0.001 0.001 0.0040.002 Nb: 0.004 Conforming steel 8 0.03 3.5 0.05 0.001 0.001 0.002 0.0010.001 0.007 0.003 — Comparative steel 9 0.04 3.3 0.06 0.001 0.050 0.0020.001 0.001 0.006 0.001 — Comparative steel 10 0.04 3.3 0.06 0.053 0.0010.002 0.001 0.001 0.006 0.001 — Comparative steel 11 0.04 3.3 0.07 0.0640.041 0.001 0.024 0.001 0.003 0.003 — Comparative steel 12 0.03 3.4 0.060.045 0.068 0.021 0.001 0.011 0.003 0.003 — Comparative steel 13 0.023.2 0.07 0.035 0.050 0.001 0.001 0.001 0.023 0.003 — Comparative steel14 0.03 3.3 0.09 0.045 0.049 0.002 0.001 0.001 0.003 0.008 — Comparativesteel

TABLE 4 Additive amount of Magnetic property of product sheet magnesiumsulfate B₈ W_(17/50) TD-H₁₀ TD-W₁₀ ΔW No. (%) (T) (W/kg) (A/m) (W/kg)(W/kg) Remarks 1 3 1.949 0.81 282 2.003 0.004 Example 2 10 1.960 0.80278 1.922 0.005 Example 3 5 1.950 0.77 285 2.022 0.003 Example 4 5 1.9540.78 288 2.101 0.002 Example 5 3 1.950 0.79 286 2.076 0.003 Example 6 31.950 0.78 278 1.983 0.005 Example 7 10 1.960 0.78 281 2.000 0.004Example 8 0 1.909 0.90 220 1.703 0.008 Comparative Example 9 0 1.9510.89 188 1.783 0.030 Comparative Example 10 0 1.905 0.93 238 1.803 0.010Comparative Example 11 0 1.830 1.45 160 1.432 0.045 Comparative Example12 0 1.802 1.77 145 1.382 0.059 Comparative Example 13 0 1.804 1.72 1381.432 0.055 Comparative Example 14 0 1.811 1.62 165 1.543 0.044Comparative Example

As is clear from Table 4, by using the material containing appropriateamounts of P and Sb in combination, applying the annealing separatorhaving MgO as a main ingredient and containing 2.0% or more magnesiumsulfate, performing secondary recrystallization annealing, and furtherapplying an appropriate flattening annealing condition according to thedisclosure, not only favorable magnetic flux density was obtained, butalso iron loss degradation caused by the application of strainassociated with processing into a transformer was reduced.

1. A grain-oriented electrical steel sheet having a compositioncontaining, in mass %, C: 0.005% or less, Si: 2.0% to 4.5%, and Mn: 0.5%or less, and also containing Sb and P in respective ranges satisfying0.01%≦[% Sb]≦0.20% and 0.02%≦[% P]≦2.0×[% Sb], with a balance being Feand incidental impurities, wherein when the steel sheet is excited to1.0 T at 50 Hz in a rolling transverse direction, a magnetizing force(TD-H₁₀) and an iron loss (TD-W₁₀) are respectively (TD-H₁₀)≧200 A/m and(TD-W₁₀)≧1.60 W/kg.
 2. The grain-oriented electrical steel sheetaccording to claim 1, wherein the composition further contains, in mass%, one or more selected from Ni: 0.005% to 1.50%, Sn: 0.03% to 0.20%,Cu: 0.02% to 0.50%, Cr: 0.02% to 0.50%, Mo: 0.01% to 0.50%, and Nb:0.002% to 0.01%.
 3. A method for producing a grain-oriented electricalsteel sheet, comprising: providing a steel slab having a compositioncontaining, in mass %, C: 0.08% or less, Si: 2.0% to 4.5%, and Mn: 0.5%or less, containing each of S, Se, and O: less than 50 ppm, N: less than60 ppm, and sol.Al: less than 100 ppm, and also containing Sb and P inrespective ranges satisfying 0.01%≦[% Sb]≦0.20% and 0.02%≦[% P]≦2.0×[%Sb], with a balance being Fe and incidental impurities; optionallyreheating the steel slab; thereafter hot rolling the steel slab toobtain a hot rolled sheet; optionally hot band annealing the hot rolledsheet; thereafter cold rolling the hot rolled sheet either once, ortwice or more with intermediate annealing performed therebetween, toobtain a cold rolled sheet having a final sheet thickness; thereafterperforming decarburization and primary recrystallization annealing onthe cold rolled sheet, to obtain a decarburization and primaryrecrystallization annealed sheet; thereafter applying an annealingseparator mainly composed of MgO to the decarburization and primaryrecrystallization annealed sheet; thereafter performing secondaryrecrystallization annealing on the decarburization and primaryrecrystallization annealed sheet, to obtain a secondaryrecrystallization annealed sheet; and further performing flatteningannealing on the secondary recrystallization annealed sheet, wherein 2.0mass % to 15.0 mass % magnesium sulfate is contained in the annealingseparator, the flattening annealing is performed at a temperature of830° C. or more in an atmosphere having a H₂ partial pressure of 0.3% ormore, and when the steel sheet is excited to 1.0 T at 50 Hz in a rollingtransverse direction, a magnetizing force (TD-H₁₀) and an iron loss(TD-W₁₀) are respectively (TD-H₁₀)≧200 A/m and (TD-W₁₀)≧1.60 W/kg.